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Heartworm Disease in Dogs and Cats

Heartworm Disease in Dogs and Cats
April 1, 2019

Canine and feline heartworm disease is a serious and deadly disease found wherever mosquitoes are present. Each year, thousands of dogs become permanently debilitated or die needlessly from this easily preventable disease.

Heartworm Life Cycle

  • The heartworm life cycle begins when a mosquito ingests a microfilaria (larval heartworm) while sucking blood from an infected animal.
    The microfilariae develop into the infective stage in the mosquito in 2-3 weeks. The infective stage is then passed to the dog when the mosquito takes a blood meal.
  • If the larva is passed into a mammal other than a dog, for example, a human, the immune system and tissues destroy the larva before it can cause disease.
  • In the dog and, rarely, the cat, the larvae find suitable conditions for survival and propagate. Over the next 2-3 months, the larvae develop and migrate to the heart.
  • The life-cycle has now run full circle 5-6 months after the dog was initially infected as the adult female heartworms begin to give birth to microfilariae.

Remember that heartworm disease is transmitted by mosquitoes! Any dog and cat can get this disease (not just animals that go outside).

Symptoms of Heartworm Disease

The symptoms of heartworm disease are related to the cardio-vascular problems it creates. Coughing, lack of energy, tiring easily and weight loss are some of the common clinical signs of advanced disease. Secondary liver and kidney disease may develop in some dogs. Heartworms average 5 to 14 inches in length and as many as 100 worms have been found in a single dog!

There are reliable tests to detect heartworm disease. Your pet should be tested once a year and put on heartworm preventative all year round.


Prevention is as simple as giving a pill once a month. We recommend Heartguard (ivermectin) for cats and dogs. Rough collies should be cautious with Heartguard they are sensitive to ivermectin. There are other forms of prevention to use instead (please consult your veterinarian).